1. Field of the Invention
The present invention relates to a head position detecting method and a disk device, and, in particular, to a head position detecting method and disk device in which a servo signal is detected in accordance with an area servo method.
Recently, as an information amount increases in an information-oriented society, increase of the storage capacity of a magnetic disk device and high-speed access to stored data are demanded. Therefore, for a magnetic disk, increase of BPI (Byte Per Inch: a unit for indicating a recording density) and improvement of TPI (Tracks Per Inch: a unit for indicating a track density) are requested.
However, in a magnetic disk device, when increasing TPI, a dead space between tracks decreases. When a dead space between tracks decreases, higher head position detecting accuracy is required.
In order to achieve higher head position detecting accuracy, it is necessary to detect a servo signal with high accuracy. For this purpose, a servo signal detecting method has been changed from a peak hold method to a so-called area servo method. In the peak hold method, the head position is detected from the peak value of a servo signal. In the area servo method, a servo signal waveform is integrated and the head position is detected from the integrated value.
2. Description of the Related Art
FIG. 1 shows a block diagram of one example of a magnetic disk device in the related art.
The magnetic disk device 1 causes magnetic heads 3 to approach magnetic disks 2, which are made of a magnetic substance and rotate in an arrow-A direction, and performs information recording and reproducing. In the magnetic disks, cylinders are previously set concentrically for fixing information recording and reproducing positions. The magnetic heads 3 are held at a projecting end of an arm 4. The other end of the arm 4 is held by an actuator 5. The actuator 5 rotates the arm 4 in an arrow-B direction about a shaft 5a, and performs position control so as to position the magnetic heads 3 at a desired cylinder.
Further, the signal reproduced as a result of the magnetic heads 3 scanning the magnetic disk 2 is supplied to a R/W (Read/Write) pre-amplifier 6. The R/W pre-amplifier 6 amplifies the signal reproduced by the magnetic heads 3, supplies the signal to an automatic gain control (AGC) amplifier 7, and, also, amplifies a recording signal to be recorded to the magnetic disks 2 and supplies the signal to the magnetic heads 3.
The AGC circuit 7 controls the amplitude of the main signal of the reproduced signal amplified by the R/W pre-amplifier 6 to be equal to or less than a fixed level. The output signal of the AGC circuit 7 is supplied to a signal detecting portion 8 and a servo detecting portion 9. The signal detecting portion 8 reads control information and data from the reproduced signal, converts it into digital information and supplies it to a CPU 10. The CPU 10 decodes the digital information read from the signal detecting portion and outputs it as reproduced data.
The servo detecting circuit 9 detects a servo portion of the output signal of the AGC amplifier 7, detects the current position of the magnetic heads 3 by a servo burst signal of the servo portion, generates an error signal and supplies it to the CPU 10.
In accordance with the error signal from the servo detecting portion 9, the CPU 10 generates a position control signal for controlling the position of the magnetic heads 3, and supplies the signal to a D/A (Digital/Analog) converter 11. The D/A converter 11 converts the position control signal supplied by the CPU 10 into an analog signal and supplies the signal to a driver 12. In accordance with the position control signal supplied by the D/A converter 11, the driver 12 generates a driving signal for driving the actuator 5, and supplies the signal to the actuator 5.
The actuator 5 rotates about the shaft 5a in accordance with the driving signal supplied by the driver 12, and rotates the arm 4 in the arrow-B direction. As a result of the rotation of the actuator 5, the magnetic heads 3 held at the projecting end of the arm 4 move in the arrow-B direction on the magnetic disks 2, and scan a desired cylinder on the magnetic disks 2.
FIGS. 2A and 2B show a data format of the magnetic disks. FIG. 2A shows a perspective view of the magnetic disk 2 and FIG. 2B shows a development of the cylinders.
As shown in FIG. 2A, the plurality of cylinders 21 are formed concentrically on the two sides of the magnetic disk 2. As shown in FIG. 2B, the servo portions 22 are formed with predetermined intervals in the cylinders 21 for recognizing the position of the magnetic heads 3 as a result of being read by the magnetic heads 3. Between the servo portions, data portions 23 for writing data thereto are formed.
At the time of recording and reproducing, the servo burst signal written in the servo portions 22 is reproduced by the magnetic heads 3, and, through the reproduced signal, the cylinder number at which the magnetic heads 3 currently scan and the position shift on the cylinder are recognized by the CPU 10.
FIG. 3 shows a data format of the servo portion of the magnetic disks.
The servo portion 22 includes an AGC portion 24 for fixing the signal reception level, a training pattern portion 25 for indicating the start of the servo information, a servo information portion 26 in which digital information such as the cylinder number and so forth are recorded and a servo burst portion 27 in which the servo burst signal for generating a tracking error signal is recorded.
As shown in FIG. 3, the servo burst portion 27 includes the servo burst signals S1, S2, which are formed to lie across adjacent cylinders, and the servo burst signals S3, S4, which are formed on respective cylinders. The servo burst signals S1, S2, S3 and S4 are formed successively in the direction in which the cylinders extend.
FIG. 4 shows a waveform of the servo burst signals reproduced by the magnetic heads. FIG. 4 shows the waveform of the reproduced signal obtained when, in FIG. 3, the magnetic head 3 scans an approximately central portion of the cylinder 21-1 in an arrow-C direction.
During the time from t0 through t1, the magnetic head 3 scans the servo burst signal S1. During this time, because the servo burst signal S1 is formed to lie across from the center of the cylinder 21-1 to the center of the cylinder 21-2, when the magnetic head 3 scans the center of the cylinder 21-1, the magnetic head 3 scans approximately half of the servo burst signal S1. Accordingly, the amplitude is approximately half in comparison to the case where the magnetic head 3 reproduces the servo burst signal with the entirety of the magnetic head 3.
Then, when, during the time from t1 through t2, the magnetic head 3 scans the cylinder 21-1 in the arrow-C direction, the magnetic head 3 scans the servo burst signal S2. During this time, because the servo burst signal S2 is formed to lie across from the center of the cylinder 21-3 to the center of the cylinder 21-1, when the magnetic head 3 scans the center of the cylinder 21-1, the magnetic head 3 scans approximately half of the servo burst signal S2. Accordingly, the amplitude is approximately half in comparison to the case where the magnetic head 3 reproduces the servo burst signal with the entirety of the magnetic head 3.
Then, when, during the time from t2 through t3, the magnetic head 3 scans the cylinder 21-1 in the arrow-C direction, the magnetic head 3 scans the servo burst signal S3. During this time, because the servo burst signal S3 is formed across the entire width of the cylinder 21-1, all of the signal reproduced by the magnetic head 3 is the servo burst signal, and the amplitude is maximum among the cases where the magnetic head 3 reproduces the servo burst signals.
Then, when, during the time from t3 through t4, the magnetic head 3 scans the cylinder 21-1 in the arrow-C direction, the magnetic head 3 scans between the servo burst signals S4. Accordingly, the servo burst signals S4 are not reproduced.
The reproduced signals of the servo burst signals as shown in FIG. 4 are supplied to the servo detecting portion 9 through the R/W pre-amplifier 6 and AGC amplifier 7. In order to accurately obtain the difference between the above-mentioned servo burst signal S1 and servo burst signal S2 which are formed across adjacent cylinders, the servo detecting portion 9 obtains the integrated values obtained from integrating the servo burst signal S1 and servo burst signal S2.
FIG. 5 shows a block diagram of one example of the servo detecting portion in the related art.
The servo detecting portion 9 includes a full-wave rectifier 31 which performs full-wave rectification on the output servo burst signal of the AGC amplifier 7, an integrating circuit 32 which integrates the servo burst signal which has undergone the full-wave rectification in the full-wave rectifier 31, an A/D converter 33 which converts the integrated value obtained from the integrating circuit 32 into the digital data, a zero-crossing detector 34 which detects the zero-crossing points of the output servo burst signal of the AGC amplifier 7 and an integration control circuit 35 which counts the zero-crossing points detected by the zero-crossing detector 34 and controls the integrating circuit 32 so that the integrating circuit 32 holds the integrated value when the count becomes a previously set count value.
FIG. 6 shows a block diagram of the integrating circuit in the related art.
The integrating circuit 32 includes a capacitor Cap which stores the servo burst signal which has undergone the full-wave rectification in the full-wave rectifier 31 and a holding circuit 36 which holds the charged voltage stored in the capacitor Cap.
The CPU 10 and the integration control circuit 35 are connected to the holding circuit 36, and the holding circuit 36 discharges the charged voltage held in the capacitor Cap and holds the charged voltage of the capacitor Cap. In response to a start control signal from the CPU 10, the holding circuit 36 discharges the capacitor Cap and charges the servo burst signal in the capacitor Cap.
The start control signal from the CPU 10 is also supplied to the integration control circuit 35 and resetting of the zero-crossing point count value is performed. The integration control circuit 35 is reset by the start control signal from the CPU 10, counting is started, and the integration control circuit 35 causes the holding circuit 36 to hold the charged voltage of the capacitor Cap when the count becomes the predetermined count value.
FIGS. 7A, 7B and 7C show an operation explanation drawing. FIG. 7A shows the servo burst signal, FIG. 7B shows the zero-crossing count value and FIG. 7C shows the charged voltage of the capacitor Cap.
When the start control signal is output from the CPU 10 at the time t0, the capacitor Cap is discharged, the charged voltage of the capacitor Cap becomes zero as shown in FIG. 7C, the integration control circuit 35 is reset as shown in FIG. 7B and the zero-crossing point-count value of the servo burst signal is caused to be zero.
Then, when the servo burst signal passes the zero-crossing point at the time t1 as shown in FIG. 7A, the count value of the integration control circuit 35 is incremented and becomes `1`. Similarly, at the times from t2 through t10, the servo burst signal crosses zero, and the count value of the integration control circuit 35 is incremented. During the time, the capacitor Cap is charged with the signal obtained from performing full-wave rectification on the servo burst signal shown in FIG. 7A, and the charged voltage thereof increases as shown in FIG. 7C.
When the count value of the integration control circuit 35 becomes the predetermined count value `10` at the time t10, the integration control circuit 35 controls the holding circuit 36 so that the charging of the capacitor Cap is stopped. Further, the integration control circuit 35 causes the holding circuit 36 to hold the charged voltage of this time. The charged voltage V1 held in the holding circuit 36 is converted into the digital data by the A/D converter 33 and is supplied to the CPU 10.
In accordance with the difference between the integrated values of the servo burst signal S1 and the servo burst signal S2, the CPU 10 generates an error signal for controlling the position of the magnetic head 3 so that the magnetic head 3 scans the center of the desired cylinder 21-1.
However, in the servo detecting circuit, using the area servo method, of the magnetic disk device in the related art, when noises occur around the zero-crossing point of the servo burst signal as indicated by broken lines around the times t11, t5 and t12 of FIG. 7A, the noises are counted as the zero-crossing points as indicated by (5) and (7) in FIG. 7B, the peak number of the burst signal to be integrated increases and the count value of the integration control circuit becomes `10` at the time t8. Accordingly, although the integration by the capacitor Cap should be stopped at the time t10, the integration is stopped at the time t8. As a result, the integrated value V2, of the integration period shorter by the time (t10-t8) than that of the integrated value V1 of the normal case, is detected.
Thus, due to the noises, variation occurs in the integrated value of the servo burst signal, the accurate on-track condition cannot be detected, the head cannot be positioned accurately, and so forth. Thus, a problem occurs.
Further, in the servo circuit, using the area servo method, of the magnetic disk device in the related art, the capacitance for charging the servo burst signal is fixed. The waveforms of the read servo burst signals are different between the case of the position of the magnetic head being on the inner side of the magnetic disk and the case of the position of the magnetic head being on the outer side of the magnetic disk.
FIG. 8 shows waveforms of the servo burst signals of the inner side and outer side of the magnetic disk.
In FIG. 8, the solid line indicates the waveform of the servo burst signal of the inner side and the broken line indicates the waveform of the servo burst signal of the outer side.
Because recording density of the magnetic disk is different between the inner side and outer side of the magnetic disk, a difference occurs in the half-value widths W.sub.50 of the reproduced servo burst signals. When recording density is maximum in the inner side, the half-value width W.sub.50 of the servo burst signal in the outer side decreases and the waveform is distorted as indicated by the broken line in the figure.
Accordingly, in the outer side of the magnetic disk, the integrated value of the servo burst signal is smaller than that of the inner side by the amount indicated by the broken-line hatching in FIG. 8. When the integrated value of the servo burst signal becomes smaller, change of the integrated value for the position of the magnetic head becomes smaller. Thereby, the difference of the integrated value of the burst signal for the change amount becomes smaller and sensitivity for position shift of the magnetic head is lowered. As a result, head positioning cannot be performed accurately and so forth. Thus, a problem occurs.